Static bioassays lasting 11 to 16 days were conducted on Chlorella pyrenoidosa using vanillic acid, syringic acid, and 4-hydroxybenzoic acid. Vanillic and 4-hydroxybenzoic acid were initially inhibitory, then after 3 to 5 days became stimulatory compared to control. Bioassays with syringic acid were all stimulatory except higher concentrations, which resulted in 100% mortality. Bacteria-free cultures of C. pyrenoidosa were stimulated by vanillic acid at low concentration and inhibited at higher levels with no shift in response observed. It was concluded that degradation of test material was responsible for the shift from inhibition to stimulation. This work concludes that the length of tests, as well as whether or not careful monitoring for bacterial contamination was conducted, can have a significant impact on bioassay results.